I'm not an expert on this, but I do know my share. Here are the links. The first one is how RDRAM measures up to SDRAM. The second is for how it measures up to DDR-SDRAM.
My gateway 700c 2.0ghz P4 PC (3 years old) came with pc-800 rdram. It works well but 256 rdram these days is like $90 on average and they have to run in identicle pairs. I think ddr2 is the best choice
DDR always came at 64-Bit...now it is doubled due to Dual Channel, but the individual bus width is still 64-Bit. RDRAM is good and PC600/16-Bit was the lowest they ever made.